Form libraries are used to control user access rights to a set of forms. Typically forms are organized into groups or individual job functions.
For example, you may want to create a Form Library called "Sales Forms" to hold sales related forms. Then you would assign only sales personnel to have access to that library.
Forms that don't belong to any Form Libraries will not be viewable by any mobile user.
You can setup as many Form Libraries as needed within the administration portal.
For more information on Libraries and their uses please check out this video https://mergemobile.zendesk.com/hc/en-us/articles/115000813306-My-Forms-Managing-Libraries-Assigning-Permissions-Editing-Forms-Form-History-and-Recovering-Deleted-Forms-Video-